Diversity techniques at the receiving end of a wireless communications link can improve signal performance without additional interference. Space diversity typically uses two or more receive antennas spatially separated in the plane horizontal to local terrain. The use of physical separation to improve communications system performance is generally limited by the degree of cross-correlation between signals received by the two antennas and the antenna height above the local terrain. The maximum diversity improvement occurs when the cross-correlation coefficient is zero.
For example, in a space diversity system employing two receive antennas, the physical separation between the receive antennas typically is greater than or equal to eight (8) times the nominal wavelength of the operating frequency for an antenna height of 100 feet (30 meters). Moreover, the physical separation between antennas typically is greater than or equal to fourteen (14) times for an antenna height of 150 feet (50 meters). The two-branch space diversity system cross-correlation coefficient is set to 0.7 for the separations identified above. At an operating frequency of 850 MHz, a separation factor of 8 wavelengths between receive antennas creates a .+-.2 dB power difference, which provides a sufficient improvement of signal reception performance for the application of the diversity technique. For a communications system operating at 850 MHz, the physical separation of the receive antennas is approximately nine feet (3 meters).
Site installation issues become increasingly impractical for lower frequency applications for which the wavelength is greater. For instance, the antenna separation required at 450 MHz is nearly 18 feet for equivalent space diversity performance assuming the same height criteria is applicable. Although the site installation issues would be relieved for higher frequencies because of the reduction in the baseline distance required for diversity performance, there is a need to reduce the physical presence of base station antennas to improve the overall appearance of the antenna within its operating environment and to improve the economics of the site installation.
Present antennas for wireless communications systems typically use vertical linear polarization as the reference or basis polarization characteristic of both transmit and receive base station antennas. The polarization of an antenna in a given direction is the polarization of the wave radiated by the antenna. For a field vector at a single frequency at a fixed point in space, the polarization state is that property which describes the shape and orientation of the locus of the extremity of the field vector and the sense in which the locus is traversed. Cross polarization is the polarization orthogonal to the reference polarization.
Space diversity antennas typically have the same vertical characteristic polarization state for the receive antennas. Space diversity, when applied with single polarization antennas, is incapable of recovering signals which have polarization characteristics different from the receive antennas. Specifically, signal power that is cross polarized to the antenna polarization does not effectively couple into the antenna. Hence, space diversity systems using single polarized antennas have limited effectiveness for the reception of cross-polarized signals. Space diversity performance is further limited by angle effects, which occur when the apparent baseline distance between the physically separated antennas is reduced for signals having an angle of arrival which is not normal to the baseline of the spatially separated array.
Polarization diversity provides an alternative to the use of space diversity for base stations of wireless communications systems, particularly those supporting Personal Communications Services (PCS) or cellular mobile radiotelephone (CMR) applications. The potential effectiveness of polarization diversity relies on the premise that the transmit polarization of the typically linearly polarized mobile or portable communications unit will not always be aligned with a vertical linear polarization for the antenna at the base station site or will necessarily be a linearly polarized state (e.g., elliptical polarization). For example, depolarization, which is the conversion of power from a reference polarization into the cross polarization, can occur along the propagation path(s) between the mobile user and base station. Multipath propagation generally is accompanied by some degree of signal depolarization.
Polarization diversity may be accomplished for two-branches by using an antenna with dual simultaneous polarizations. Dual polarization allows base station antenna implementations to be reduced from two physically separated antennas to a single antenna having two characteristic polarization states. Dual polarized antennas have typically been used for communications between a satellite and an earth station. For the satellite communication application, the typical satellite antenna is a reflector-type antenna having a relatively narrow field of view, typically ranging between 15 to 20 degrees to provide a beam for Earth coverage. A dual polarized antenna for a satellite application is commonly implemented as a multibeam antenna comprising separate feed element arrays and gridded reflecting optics having displaced focal points for orthogonal linear polarization states or separate reflecting optics for orthogonal circular polarization states. An earth station antenna typically comprises a high gain, dual polarized antenna with a relatively narrow "pencil" beam having a half power beamwidth (HPBW) of a few degrees or less.
The present invention provides the advantages offered by polarization diversity by providing antenna having an array of dual polarized radiating elements arranged within a planar array and exhibiting a substantially rotationally symmetric radiation pattern over a wide field of view. In contrast to prior dual polarized antennas, present invention maintains the substantially rotationally symmetric radiation pattern for HPBW within the range of 45 to 120 degrees. A high degree of orthogonality is achieved between the pair of antenna polarization states regardless of the look angle over the antenna field of view. The antenna dual polarizations can be determined by centrally-located polarization control network, which is connected to the array of dual polarized radiators and can accept the polarization states of received signals and output signals having different predetermined polarization states. The antenna of the present invention can achieve a compact structure resulting in low radio-electric space occupancy, and is easy and relatively inexpensive to reproduce.